A method of this kind is known in the marketplace wherein fuel is injected into an intake manifold via an injection valve. In other known methods, the fuel is injected directly into the combustion chamber. The known method is applicable to gasoline as well as to diesel. The injected fuel becomes a mist in the intake manifold and is transported into a combustion chamber of the engine. The fuel is supplied to the injection valve from a fuel feedline which is under an essentially constant pressure. If, for example, a high load is requested by the driver, the injection valve is so driven that it remains open for a relatively long time. In this way, a larger quantity of fuel enters the intake manifold.
The maximum possible injection duration of the injection valve is, however, limited. If, for example, in gasoline-direct injection, a discrete opening operation of the injection valve is wanted, then the maximum possible injection duration is equal to the duration of the induction stroke plus, if required, the compression stroke. For intake manifold injection, the injection valve can, in the extreme case, also be open continuously. However, it would be more advantageous to have here also a discrete opening operation because injection should not take place into an open inlet valve. Accordingly, the injection valve should be closed when the inlet valve is open.
If the operator of the engine requests a high load (that is, for example, a high torque with a possibly high rpm), it can happen that the requested injection duration of the injection valve is longer than the maximum possible injection duration. The maximum possible injection is, however, limited by the engine. For this reason, only a fuel quantity is injected which corresponds to the maximum possible injection duration notwithstanding this higher requested injection duration. At the same time, an air charge is supplied to the combustion chamber which corresponds in quantity to the requested high torque so that the mixture, which enters the combustion chamber, is overall too lean. This leads, on the one hand, to a poor emission performance and, on the other hand, to an increased temperature of the engine and the exhaust gas.
High exhaust gas temperatures, in turn, are damaging for the exhaust-gas system. Furthermore, exhaust gas temperatures, which are too high, are especially unwanted when the engine has a compressor (for example, a turbocharger), which precompresses the intake air in specific operating states. The turbine of the turbocharger disposed in the exhaust-gas flow is exceptionally temperature sensitive and can be damaged by an exhaust-gas temperature which is increased to this extent. For this reason, especially in the full-load range, an enrichment of the mixture takes place in order to reduce the exhaust-gas temperature. This is not possible when the engine is already operated in the state wherein the injection duration can no longer be extended.
The use of a fuel having a lower octane number is also made more difficult because, in this case, the engine should be operated with a retarded ignition angle in order to avoid knocking. However, this retarded ignition angle causes the exhaust-gas temperature to be increased. This would have to be countered with an enrichment of the mixture, which is not possible when the injection valve is already operated at the maximum possible injection duration.
In order to avoid the above-mentioned problems, a most precise design of the injection valve is required. This must take place so that it is ensured in each operating state that an adequate fuel quantity can be injected. This leads, in general, to the demand for an injection valve which can inject a relatively large quantity of fuel. During idle of the engine, it is, however, required that the injection duration of the injection valve be only very short. The injection valve must supply only a very low quantity of fuel to the combustion chamber of the engine, especially for an active tank venting where fuel vapor is conducted directly from the tank into the intake manifold. In order to properly consider all these requirements, the injection valve must therefore have a very large variation range, that is, it has to be able to inject the smallest quantities of fuel but also large quantities of fuel. Injection valves of this type are, however, very expensive.